A conventional digital content protection system enabling legally copying and communicating copyrighted digital content is taught, for example, in Japanese Patent 3380194. When a device (“transmitting device” below) that first transmits the digital content in this system copies and sends the digital content to another device (the “receiving device”), the transmitting device encrypts the digital content using a unique key and records the encrypted digital content to a memory card storing the same unique key. The memory card is then sent to the receiving device. The receiving device then reads the unique key from the memory card and decrypts the encrypted digital content stored in the memory card using this unique key.
FIG. 5 describes the legal copy and transmission of copyrighted digital content using the conventional digital content protection system taught in Japanese Patent 3381094.
As shown in FIG. 5, this digital content protection system includes a memory card 10, a transmitting device 20b, and a receiving device 30b. When copying digital content, the internal unique key 11 is read from the memory card 10 and encrypted, and then sent as information 111 to the transmitting device 20b. The authentication unit 21b of the digital content transmitting device 20b then decrypts the received information 111 to acquire the unique key 11. Only authentic transmitting devices know the correct method of decoding the information 111, and the transmitting device can thus be verified. The content processing unit 22b then encrypts the copyrighted digital content 23b using the decrypted unique key 11, and then records the resulting encrypted content 231 to the digital content recording area 12 of the memory card 10. The recorded digital content 23b is then sent with the memory card to which it is stored to the receiving device 30b. 
The receiving device 30b then reads the encrypted unique key information 111 from the memory card 10, and the receiver-side authentication unit 21b decrypts and acquires the unique key 11. The decryption unit 32b reads and decrypts the encrypted content from the digital content recording area 12 using the acquired unique key 11, and outputs the decrypted content.
The unique key 11 stored in the memory card cannot be read directly, and can only be read as encrypted data 111. Therefore, if the receiving device 30b is unauthorized, it cannot correctly decode the encrypted data 111, and unauthorized copy of digital content can be prevented.
Furthermore, when digital content is recorded to an unauthorized memory card 10 that does not have the unique key, the authentication unit 21b of the transmitting device 20b is unable to verify the memory card 10, and illegal copying is therefore again prevented.